I don't have a specific reference for this, but I'd suggest a simple answer: Electronic Counter Measures (ECM)
We know that both the Colonials and the Cylons use electronic warfare. The Raptor, in particular, is noted on many occasions to carry an ECM suite. ECM is a broad array of tools that work along similar principles - to blind, jam, or otherwise trick enemy electronic detectors.
This may involve releasing chaff, or strips of radar-reflecting metal, that create false targets on enemy dradis; decoys, small drones that radiate a signal that matches the enemy's dradis frequency; or various forms of jamming, where the ECM system floods the local space with radio noise in specific frequencies that hide the dradis returns (in your first image, the manoeuvring missile had probably been affected by ECM - it thought it had a target, manoeuvred towards it, lost it, manoeuvred again...etc).
While ECM keeps your enemy's missiles from clearly seeing where your ships are, you can also move in unpredictable courses or jink. Especially when battles are fought in space and at long range, the spot you fired at a moment ago may not be the spot the enemy is in when the shot reaches it.
So if you're fighting at long range, your target could be jinking around so you're not sure where they're going to be at any given moment, AND they're degrading your tracking signals with ECM, what's a genocidal species of cripplingly insecure robots to do?
Fire everywhere the enemy might be, that's what.
Instead of saving resources by trying for precision hits with a few super-accurate missiles, you fire everywhere. In front of the enemy, behind him, above him, below him, and straight at him. You box him in, straddle him with fire, so that no matter which missiles turn out to be in the right place, something's going to hit. It's the same principle as fully automatic rifle fire; you don't expect every round to hit, but you've got rounds to burn so you may as well shove some everywhere someone might be.